One method for measuring the moisture content of a medium is by application of sensors, by means of which the moisture content is expressed by an electric signal. At present, known sensors for measuring moisture content are made with lithium chloride which is applied with a suitable binding agent on a support. Other known sensors are made with powder carbon, with layers of different polymers or manufactured on the basis of porous aluminium oxide.
Sensors with lithium chloride and with powder carbon and with polymers are sensitive to moisture due to variations of their volume in different layers. Their drawback is however a long reaction time to changes of moisture, the instability of their calibration curve and their high dependence on the temperature of the sensor.
A sensor with a thin porous layer of aluminium oxide where water is absorbed on the surface of pores of its layer causing thereby a change of electric parameters of the sensor, offers more advantageous properties. In this case surface layers are concerned and the reaction time is substantially shortened. The prolonged calibration stability which can be achieved and the resulting value of electric impedance of these sensors depend on the technology which has been applied for preparation of the layer of aluminium oxide and on its further treatment.
Good results have been obtained with sensors manufactured according to U.S. Pat. No. 3,075,385 where the porous layer of aluminium oxide is formed on the surface of a clean, polished aluminium sheet which forms simultaneously a main electrode of the sensor, by anodic oxidation by alternating current having a density of about 13 mA/cm.sup.2 in 50% sulphuric acid at a temperature of 33 degrees C. for 35 minutes. The thus obtained layer of a thickness of 2 .mu.m is, just after rinsing with distilled water, exposed to a hydratation in boiling redistilled water, to the so-called sealing, which proceeds for about 30 minutes. The surface of the porous aluminium oxide is covered in the course of this process by a thin layer of aluminiumoxihydroxide, by the so-called boehmite. A thin semipervious counter electrode is created by a suitable method on a part of the thus prepared composite layer. The obtained condensor structure reacts to a change of the surrounding moisture by a change of electric parameters, which changes are evaluated. Sensors manufactured acording to this process are sensitive within a wide range of moisture content and have a relatively short reaction time, their drawback is however that, due to absorption of water to the porous layer, a further consecutive hydratation is caused leading to non-recurring changes of the structure of the sensor and to a reduction of its sensitivity.
A process described in U.S. Pat. No. 3,523,244, which is a modification of the above-mentioned process, uses a layer of aluminium oxide, the thickness of which is, after hydratation, reduced by mechanical grinding to about 300 nm, due to which reduction a reduction of the reaction time in the sensor is achieved.
The above-mentioned drawbacks are much reduced by a manufacturing method described in Czechoslovak certificates of authorship of inventions No. 197 742 and 210 174 and in U.S. Pat. No. 4,441,968, where the surface of aluminium oxide is, prior to application of a counter electrode, passivated for instance by soaking in an aqueous solution of 0.1 mol/liter sodium-hydrogenphosphate at 90 degrees C. for a time of 10 minutes with a following tempering in a gaseous medium which is chemically inert against the thus created layer at a temperature of 90 degrees C. for 8 hours. It is suitable if this medium contains water vapors of a lower pressure than the aqueous water tension at the tempering temperature.
Although the thus prepared sensor has a very good calibration stability, its drawback is that its manufacturing process is rather complicated as it contains a number of different operations. That leads generally to a more frequent occurrence of faulty products, to a dissipation of resulting parameters of individual sensors and to certain variations of the shape of their calibration curves.
It is an object of this invention to provide a method of manufacturing an electric moisture-content sensor which eliminates or reduces the above mentioned drawbacks.